Coral Spawning Events
Before getting into what happened, let me
first say a few things about spawning events.
To start, there are several environmental cues
that can elicit a mass spawning on a reef.
Factors like the species present, their location,
fluctuations in water temperature and salinity,
seasonal changes in the length of days, and
changes in the tides and lunar phases can
all play a role. There isn’t a single factor that
brings about mass spawning events.
Regardless, when mass spawning events
occur, large numbers of corals eject huge
numbers of gametes (sperm and eggs) into
the surrounding waters all at about the same
time. Sperm can be pumped out in clouds,
and eggs may be ejected singly. However,
in many cases, clumps of eggs are ejected,
and these clumps have a package of sperm
tucked in the middle of them. These gamete
bundles come apart over time, and the eggs
and sperm spread. Ideally, the sperm cells
eventually run into eggs from another coral
in the area and fertilize them.
Unfortunately, when this occurred in my
aquarium, I didn’t really see much of it. By
the time I realized the corals had spawned,
they were pretty much done. I could see
several types of gamete bundles drifting
around, and a lot of sperm, which just made
the water cloudy, but there was no way to
tell exactly what spawned and what didn’t,
although the volume of gametes made it
look like just about everything did.
Why it happened is quite a mystery, too.
I’m still stumped, as was everyone I talked
to about it, because the tank receives no
natural light and there were no changes in
temperature or salinity. It did occur just a
day after a full moon, though. On the reef,
spawning on a full moon night, or the
nights immediately following a full moon, is
common, but the moonlight is supposed to
be the trigger. Well, there was no moonlight
hitting the tank, so that means this was just
a coincidence or that somehow, someway,
the corals were affected by the moon’s
gravitational pull rather than its reflected
light. A flat-screen TV was on before and
during the event, as was a lamp, but that
is the case on many nights. Like I said, I’m
still stumped. This had never happened
before and hasn’t happened at anything
near the same scale since, either.
Regardless of the reason, the mass
spawning occurred late Sunday night into
early Monday morning, and I’ve written up
the approximate timeline with the details.
Keep in mind that I was taken completely
by surprise and unprepared to deal with the
volume of gametes released.
Sunday, 10 p.m.: A neighbor came over
to hang out and watch TV. The aquarium is
located behind my couch and not in sight
when sitting on the couch and facing the
TV. However, we were both moving around
for bathroom breaks, trips to the kitchen,
etc., and we saw nothing unusual with
respect to the aquarium. I work a somewhat
shifted schedule and typically stay up until
1 or 2 a.m., so the aquarium lights are timed
Midnight: The four daylight bulbs turned
off, but the six actinic bulbs were still on.
All of the blinds in the room were closed as
usual for nighttime, and the only other lights
in the room were the TV and a low-wattage
torchiere lamp next to the couch.
Monday, 1 a.m.: The six actinic bulbs
turned off, and my neighbor left soon after.
I cleaned up a little and flicked through
the channels for a while. Neither of us
had noticed anything, and everything was
normal, it seemed.
Monday, 2 a.m.: It was time to get to bed,
so I turned off the TV, went to the kitchen,
Tropical Fish Hobbyist www.tfhmagazine.com 25
A brain coral releasing eggs; mass coral spawning events may be triggered by environmental
cues such as fluctuations in water conditions, seasonal changes in sunrises and sunsets, and
the lunar cycle.
Here you can see the assortment of cnidarians that were present in the author’s tank when the
spawning event occurred.
The right side of the author’s aquarium,
photographed minutes after the spawning